Since you can get in a stretch limousine, and press 2, and it puts a taxi sign on top, and you can go on-duty as a taxi driver, why not expand this to all vehicles, and have a location for going on/offduty?
This would make being a taxi driver more common, and more useful. What good is a taxi if they're slow, and far away? It could work somewhat like the mechanic job, but with a different location. Maybe somewhere downtown in every city, or at a parking lot full of taxis? And then, you could also have the /jv command to let employees spawn a normal taxicab.
I think people would call for taxis way more if they came in sports cars, too, so it would make the job more popular, and more profitable.